I am using Mac OS X 10.6.5 and I have "English" set as my language in System Preferences > Language & Text > Language, but "Czech Republic Koruna" as currency in System Preferences > Language & Text > Formats. A call to setlocale(LC_ALL, "") results in NULL return value. perror then prints in log:

Based on the last line, it looks like setlocale fails because it does not find this directory (which really does not exist): /usr/share/locale/en_US@currency=CZK. The directory /usr/share/locale/en_US does exist.